1. Field of the Invention
The present invention relates to the disposal of industrial waste streams, and more particularly but not by way of limitation, to an improved process for disposing of industrial waste streams containing chemically bound nitrogen.
2. Brief Statement of the Prior Art
Oxides of nitrogen and principal contaminants emitted by process combustion of industrial waste gas streams. In every combustion process, the high temperatures at the burner result in the fixation of some oxides of nitrogen. These compounds are found in stack gases mainly as nitric oxide (NO), with lesser amounts of nitrogen dioxide (NO.sub.2) and traces of other oxides. Since nitric oxide continues to oxidize to nitrogen dioxide in air at ordinary temperatures, there is not a way to predict with accuracy the amount of each compound separately present in vented gases at a given time. Thus, the total amount of nitric oxide plus nitrogen dioxide in a sample is determined and referred to as "oxides of nitrogen", sometimes designated as nitrogen oxides or NO.sub.x.
Emissions of nitrogen oxides from stack gases, through atmospheric reactions, produce "smog" that stings eyes and causes acid rain. For this reason, the amount of nitrogen oxides present in gases vented to the atmosphere is regulated by various state and federal agencies.
There have been considerable efforts in the art to find ways to remove or prevent the formation of nitrogen oxides in waste gases so that the waste gases may be discharged to the atmosphere without harm to the environment. One prior art process for removing nitrogen oxides from waste streams utilizes an absorption medium to absorb the oxides of nitrogen. However, this method results in the formation of either an acidic liquid or other nitrogen containing noxious liquid streams which must be treated further before they can safely be discharged into the environment.
Other processes heretofore known for removing NO.sub.x from, or preventing the formation of NO.sub.x in, waste gas streams so that such waste streams can be safely vented to the atmosphere have employed catalysts in combination with reducing gases to selectively remove NO.sub.x from gaseous streams. Still other processes have employed ammonia, ammonium formate, ammonium oxalate, ammonium carbonate and the like for selectively reducing the NO.sub.x in gaseous streams.
Another prior art process for reducing the amount of NO.sub.x in industrial waste gases employs the concept of reducing the nitrogen oxides in the presence of an excess of a hydrocarbon at elevated temperatures. Such a process effectively reduces the amount of NO.sub.x present in the waste gases to a level that is satisfactory for release to the atmosphere. However, by-products, such as carbon monoxide, hydrogen and particulate carbon, are produced in such quantities that the release of the waste gases containing these by-products is harmful to the environment unless additional steps are taken to further treat the waste gases.
One process employed to reduce the amount of NO.sub.x in gaseous streams so that the gaseous streams can be vented to the surrounding atmosphere without contamination of the environment is disclosed in U.S. Pat. No. 3,873,671, issued to Reed et al. and entitled "Process for Disposal of Oxides of Nitrogen". The Reed process provides for the burning of a hydrocarbon fuel with less than the stoichiometric amount of oxygen. The oxygen may be supplied by air, or by a stream of air containing oxides of nitrogen. The combustion products of the hydrocarbon fuel are then mixed with gases to be treated containing NO.sub.x in a ratio which provides an excess of oxidizable material, under conditions that enable a portion of the combustible products to be oxidized by oxygen made available from the decomposition of the NO.sub.x, thus reducing the NO.sub.x content. This combined combustion mixture of nitrogen and other compounds, i.e., carbon monoxide, hydrocarbons, and other oxidizable materials, is thereafter cooled to a temperature in the range of from about 2000.degree. F. to about 1200.degree. F. with a cooling fluid which is substantially free of oxygen. To prevent venting excess combustibles into the atmosphere, the cooled mixture of nitrogen, combustion products and other oxidizable materials is thereafter mixed in a second zone with sufficient oxygen to convert substantially all of the oxidizable combustion products remaining to carbon dioxide and water while minimizing the reformation of oxides of nitrogen.
The process set forth in U.S. Pat. No. 3,873,671 possesses several inherent limitations. For example, the process is limited in that the reduction of the oxides of nitrogen occurs in the temperature range of about 2000.degree. F. to 1200.degree. F. In this temperature range, certain hydrocarbon fuels such as fuel oils heavier than No. 2 fuel oil, aromatic compounds, unsaturated hydrocarbons, and long chain hydrocarbons generally, will not be completely oxidized to combustible components, and the subsequent decomposition of nitrogen oxides to nitrogen will not be strongly favored. Further, if the combustible gases are cooled below about 1400.degree. F., the oxidation of these combustibles by oxygen in the second oxidation zone may not be adequately achieved because the combustible components are greatly diluted by nitrogen, carbon dioxide and water vapor. As a result, if these combustible components are not fully oxidized due to this dilution, carbon, carbon monoxide and hydrogen will be present in the vent gases. And if these combustible vent gases are recycled to quench the reducing zone and/or the second oxidation zone, combustion can occur within the recycle stream and subsequently in the stack. Such occurrences are extremely hazardous, and can result in damage to equipment, creating dangerous conditions to operating personnel.
Another process employed to reduce the amount of NO.sub.x in gaseous waste streams is that disclosed in Japanese Patent Application No. Showa 54-50470, published Apr. 20, 1979. In this process a primary fuel is initially burned to produce a waste gas contianing NO.sub.x and at least 1% excess oxygen; a secondary light petroleum fuel, such as a light hydrocarbon or kerosene, is then introduced into the primary combustion gases to reduce the NO.sub.x therein to elemental nitrogen and more excessively reduced forms of nitrogen such as HCN and NH.sub.3 at a temperature of 1300-1800 F.; and these excessively reduced nitrogen compounds are then reoxidized back to elemental nitrogen in one or more third stages with an oxygen-containing gas.
The above-described Japanese system is cumbersome, inefficient, complicated and difficult to control, for it overreduces the nitrogen present and then corrects this over-reduction by re-oxidation in an extra stage.
While there thus have been considerable efforts to find effective ways to remove, or prevent the formation of, nitrogen oxides in waste gases so that the waste gases can be discharged into the atmosphere without harm to the environment, new and improved processes are constantly being sought which will eliminate the deficiencies of the prior art processes, which are safe to operate, and which meet the increasingly stringent regulatory requirements being placed on vented waste gases by federal and state agencies.